Rising CO2 levels could upset the delicate relationship between the butterflies and their parasites.

Marjorie Miller, July 11, 2018 – AP

Most people know that monarch butterflies can’t exist without milkweed. As caterpillars, the monarchs feed on milkweed plants exclusively, absorbing the milkweed’s poisons in order to ward off birds and other predators. On their epic migration across the North American continent, the butterflies also lay their eggs on these plants, relying on the noxious taste of the leaves to keep their brood safe from grazers while simultaneously providing a buffet for the next generation when it hatches.